Re: block cache replacement strategy?

[Johannes Stezenbach <js@sig21.net>]

On Tue, Sep 07, 2010 at 03:34:29PM +0200, Johannes Stezenbach wrote:
> 
> during some simple disk read throughput testing I observed
> caching behaviour that doesn't seem right.  The machine
> has 2G of RAM and AMD Athlon 4850e, x86_64 kernel but 32bit
> userspace, Linux 2.6.35.4.  It seems that contents of the
> block cache are not evicted to make room for other blocks.
> (Or something like that, I have no real clue about this.)
> 
> Since this is a rather artificial test I'm not too worried,
> but it looks strange to me so I thought I better report it.

C'mon guys, please comment.  Is this a bug or not?
Or is my question too silly?


Johannes

> zzz:~# echo 3 >/proc/sys/vm/drop_caches 
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.9454 s, 75.2 MB/s
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 0.92799 s, 1.1 GB/s
> 
> OK, seems like the blocks are cached. But:
> 
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.8375 s, 75.8 MB/s
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.8429 s, 75.7 MB/s
> 
> Even if I let 15min pass and repeat the dd command
> several times, I cannot see any caching effects, it
> stays at ~75 MB/s.
> 
> zzz:~# cat /proc/meminfo 
> MemTotal:        1793272 kB
> MemFree:           15216 kB
> Buffers:         1378820 kB
> Cached:            20080 kB
> SwapCached:            0 kB
> Active:           792720 kB
> Inactive:         758832 kB
> Active(anon):      91716 kB
> Inactive(anon):    64652 kB
> Active(file):     701004 kB
> Inactive(file):   694180 kB
> 
> But then:
> 
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 5.23983 s, 200 MB/s
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 0.908284 s, 1.2 GB/s
> 
> zzz:~# cat /proc/meminfo 
> MemTotal:        1793272 kB
> MemFree:           16168 kB
> Buffers:         1377308 kB
> Cached:            20660 kB
> SwapCached:            0 kB
> Active:          1140384 kB
> Inactive:         410236 kB
> Active(anon):      91716 kB
> Inactive(anon):    64652 kB
> Active(file):    1048668 kB
> Inactive(file):   345584 kB
> 
> 
> And finally:
> 
> zzz:~# echo 3 >/proc/sys/vm/drop_caches
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.948 s, 75.2 MB/s
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 0.985031 s, 1.1 GB/s
> 
> 
> Now these blocks get cached but then the others don't:
> 
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.9394 s, 75.2 MB/s
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.9403 s, 75.2 MB/s